Ensure platform-build performs git clone again if BUILD_TAG changes

If rebuilding platform-build after pushing a new version of a branch (e.g. squashed or with new commits), incr.sh will spot the new SHA and the platform-build Dockerfile will attempt to fetch it. However, the git clone will not be rerun (CACHED) and the git checkout will fail.

The Dockerfile should be changed so that if the BUILD_TAG change, the git clone is re-performed.